The invention relates to the general field of low dielectric constant layers for use in integrated circuits with particular reference to black diamond films and methods for reducing the flat band voltage.
The internal dimensions within integrated circuits continue to shrink, including the thicknesses of the dielectric layers used to separate various layers of wiring from one another. However, as these wiring levels are brought closer together, the possibility of cross-coupling between them starts to rise as well as the incidence of parasitic capacitances. One way to minimize this problem is to reduce the dielectric constants of these inter-metal layers. Thus, there is considerable interest in developing low k materials as well as deposition methods for them that are compatible with integrated circuit technology.
For our purposes we will define a low k dielectric as one that has a dielectric constant close to or less than about 3. Several such materials are known to exist but they have the disadvantage that they are organic rather than inorganic compounds. These materials are innately soft, which physical property can give rise to problems during semiconductor processing, particularly during planarization, by chemical mechanical polishing (CMP).
The present invention is concerned with low dielectric constant materials that are inorganic in nature, such as spin-on glass (SOG), fluorinated silicon glass (FSG) and, particularly, methyl-doped porous silica which is referred to by practitioners of the art as black diamond, or BD. When formed as will be described below, about 36% of a BD layer""s volume is in the form of pores having a diameter between about 8 and 24 Angstroms.
The low k films to which the present invention is directed are those that are deposited by means of PECVD (plasma enhanced chemical vapor deposition), of which black diamond is a prime example. A particular problem associated with this mode of deposition for dielectrics is the trapping of charge during their formation. The effects of this show up by their influence on the flat band voltage. This is a quantity derived from a capacitance-voltage curve of the dielectric in question. An example of this, for a typical black diamond layer formed according to the teachings of the prior art is shown in FIG. 1.
As seen in FIG. 1, the capacitance of the layer decreases with voltage along section 11 of the curve until a value of 73 volts is reached, at which point the curve becomes relatively independent of voltage (section 12). The voltage at which this change occurs is the flat band voltage and it is a measure of the charging tendency of the film. This closely correlates with the leakage current through the dielectric so it is important to reduce the flat band voltage to a value as close to zero as possible.
Dual damascene structures have received widespread application in recent years so it is important that processes for laying down low dielectric constant materials be compatible with such structures. An example of a dual damascene structure is schematially illustrated in FIG. 2. Seen there is a substrate 21 over which dielectric layer 25 has been deposited. Via hole 24 extends from the bottom of trench 23 (long dimension running normal to the plane of the figure) down to the level of substrate 21. Via and trench were over-filled with metal (usually copper) and then the upper surface was planarized, as shown.
One of the problems associated with inorganic low k films, deposited according to the teachings of the prior art, is a tendency to form cracks such as 22 as a byproduct of CMP during the formation of damascene structures. Some of these cracks may, on life, extend all the way through to the substrate. The present invention teaches a process whereby films that are immune to crack formation during CMP, as well as possessing very low flat band voltages, may be formed.
A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,593,741 Ikeda shows a low K CVD process that varies the RF power during deposition and flows oxygen gas. See abstract and col. 4 lines 30-44. U.S. Pat. No. 5,514,624(Morozumi) shows a low k oxide layer using an organosilicate reactant and an oxygen flow. In U.S. Pat. No. 5,432,129 Hodges shows an oxygen densification step for an oxide. Reference to xe2x80x9cBlack Diamondxe2x80x9d can be found on the Applied Materials website for Mar. 14, 2000. Also, in U.S. Pat. No. 4,992,306, Hochberg et al. show a PECVD SiO2 process.
It has been an object of the present invention to provide a process for depositing, through plasma enhanced chemical vapor deposition, an inorganic film of low dielectric constant.
Another object of the invention has been that said film be of high quality and reliability.
Yet another object of the invention has been that said film exhibit very low flat band voltages.
A further object has been that said film be fully compatible with use within a damascene process, particularly when chemical mechanical polishing is used.
A still further object has been that said film be free of cracks and have good adhesion, particularly after chemical mechanical polishing.
These objects have been achieved by alternating deposition of the film between low and high power modes. After deposition under low power for a few seconds the power is raised to high for a few seconds and so on, going back and forth between low and high power until the desired thickness of the film is reached. Additionally, for the deposition of materials such as black diamond, oxygen is added to the plasma during the high power phase (and removed during the low power phase). We have found that films deposited in this way have low flat band voltages, close to zero, and are, in general, more robust that films deposited according to prior art methods. In particular, these films are free of the cracking problems often encountered during chemical mechanical polishing of prior art films.